Circumferential seals are used, for example, in gas turbine engines to prevent leakage of fluid along the engine's rotating shaft where the shaft extends through a wall or partition. Referring to FIG. 1, a typical circumferential seal includes a rotating component called a seal rotor 20 and a non-rotating component called a seal stator 31. The rotor 20 is made of metal and is mounted to a rotating shaft 12. It also has a radially, outward facing sealing surface 21. The seal stator 31 includes a metal ring 35 mounted to the housing 34 and a carbon sealing ring 36 mounted to its radial inward facing surface. The stator 31 and rotor 20 are arranged so that the carbon ring 36 circumscribes the sealing surface 21 so as to seal a leakage path represented by arrow 38. To avoid damage to the carbon ring 36, a small radial gap is maintained between the ring 36 and sealing surface 21.
A common problem associated with these seals occurs as a result of variation in the radial gap between the carbon ring 36 and sealing surface 21. This variation is due in part to the mechanical growth of the rotor 20 due to centrifugal effects, but more significantly due to a disparity in thermal growth between the metal rotor and the carbon ring in response to changes in temperature. This disparity results from the two components having different coefficients of thermal expansion. The variation in the radial gap produces undesirable effect either when the radial gap is too wide open, or if it is allowed to completely close.
If the gap becomes too large, the amount of leakage through the seal increases resulting in reduced efficiency. In addition, the increased flow can adversely affect the control of pressures in neighboring cavities and hamper the intended use of the high-pressure air therein. However, if the gap is too small then substantial contact between the carbon ring and rotor can occur which can quickly damage either or both components.
One proposal for improving seal performance is to make the seal rotor from titanium, which has one of the lowest thermal expansion coefficients of any metal, and additionally satisfies strength requirements for a seal rotor. The differential thermal growth between a titanium rotor and carbon ring is substantially less than that of a seal with a more conventional nickel or iron based alloy rotor, however, it is not reduced enough to significantly improve seal performance. This is primarily due to the fact that although the thermal expansion coefficient of titanium is low for a metal, it is still much higher than that of carbon. Further, the titanium is substantially less durable than conventional rotor alloys, and thus more susceptible to damage upon contact with the stator.
Another proposal is to actively cool the rotor. A seal rotor can be cooled by providing a flow of cooling oil over its inside surfaces. This has the beneficial effect of reducing the rotor's temperature, and correspondingly reducing its thermal growth. By actively controlling the rotor's thermal growth in this way, the differential growth between the stator and rotor can be minimized. One disadvantage to an active cooling system is the added design complexity required for providing the means to deliver the oil to the runner, and the additional costs associated with that complexity. Another disadvantage is an increased risk of contamination of the air side of the seal due to the additional supply of oil in close proximity to the seal interface.
Accordingly, a need exists for a circumferential seal having a seal rotor with adequate mechanical properties and low enough thermal and mechanical growth during engine operation so that the rotor closely tracks the thermal growth of the carbon ring without the use of external cooling, and so that damage due to contact of the carbon ring and rotor is minimized.